Code

class MyStructuredType {
    Q_GADGET
    QML_VALUE_TYPE(myStructuredType)
    QML_STRUCTURED_VALUE

    // NOTE: The constructor and setters print debug messages about their parameters
    Q_PROPERTY(int number READ number WRITE setNumber FINAL)
    Q_PROPERTY(bool truth READ truth WRITE setTruth FINAL)

public:
    Q_INVOKABLE MyStructuredType(int number = 0, bool truth = false);
    Q_INVOKABLE QString toString() const;
    
    ...
}

import QtQuick
import ValueTypeTest

Window {
    width: 640
    height: 480
    visible: true

    property myStructuredType t: ({
        number: "42",
        truth: "false",
        nonexistentProp: "Oops"
    })

    Component.onCompleted: {
        console.log("=== Component Complete ===")
        console.log(t)
    }
}

Outcomes

Tooling:

• qmlsc does warn that "MyStructuredType has no property called nonexistentProp [compiler]". However, this is easily missed since qmlsc is currently silent by default.
• qmllint and qmlls don't warn about nonexistentProp

Runtime:

...
Setting number: 42
Setting truth: true
qml: === Component Complete ===
...
qml: myStructuredType(42, true)

The implicit conversion of "42" to 42 and "false" to true (for passing to the property setter functions) match the documented type coercion rules for JavaScript. However, this can be quite unintuitive.

Suggestions

• Let qmllint and qmlls warn about non-existent properties too
• Let the tools offer warnings about implicit type coercions (where type mismatches occur), even if the type coercion rules are well-defined.
  • Given that "flexibility" is an inherent quality of JavaScript, perhaps this should be opt-in and only limited to QML constructible/structured values